1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus for applying the power of an electric motor as an assistive steering force to a mechanical steering system to reduce the manual force required to steer a motor vehicle which incorporates such an electrically operated power steering apparatus.
2. Description of the Prior Art
There has been known an electrically operated power steering apparatus which comprises a steering torque sensor for detecting the steering torque of a steering system, a steering rotational speed sensor for detecting the steering rotational speed of the steering system, and a controller for determining a motor control signal to control an electric motor to generate an assistive steering force, on the basis of an output signal from the steering torque sensor and an output signal from the steering rotational speed sensor. One example of the known electrically operated power steering apparatus is disclosed in Japanese laid-open patent publication No. 62-238165 which corresponds to U.S. Pat. No. 4,730,686.
FIG. 4 of the accompanying drawings schematically shows in block form a conventional control system for controlling an electrically operated power steering apparatus which is incorporated in a motor vehicle. When a steering wheel coupled to a steering shaft is angularly moved by the driver of the motor vehicle, a steering torque sensor 13 detects a steering torque corresponding to the manual steering force applied by the driver, and outputs a steering torque signal T indicative of the detected steering torque to a controller 15.
A steering rotational speed sensor 14 detects a steering rotational speed corresponding to the speed at which the steering shaft rotates, and outputs a steering rotational speed signal .theta.' indicative of the detected steering rotational speed to the controller 15.
Based on the steering torque signal T and the steering rotational speed signal .theta. which are supplied, the controller 15 generates a motor control signal Do and supplies the motor control signal Do to a motor driver 16.
The controller 15 has a torque control quantity generator for generating a motor control quantity D.sub.T corresponding to the steering torque signal T and a rotational speed control quantity generator for generating a motor control quantity D.sub.N corresponding to the steering rotational speed signal .theta.'. The controller 15 produces the motor control signal Do by adding, subtracting, or combining the motor control quantity D.sub.T and the motor control quantity D.sub.N. The motor control signal Do is applied to control the motor driver 16 to energize an electric motor 11 with a motor drive signal Mo. The electric motor 11 produces an assistive steering force which is applied reduce manual steering forces required to steer the motor vehicle.
In the conventional control system, the steering rotational speed sensor is associated with the steering shaft for directly detecting a steering rotational speed and outputting a steering rotational speed signal .theta.' representing the detected steering rotational speed.
The steering rotational speed sensor combined with the steering shaft comprises a photoelectric transducer or a generator, and hence occupies a relatively large space around the steering shaft. Therefore, the steering rotational speed sensor poses a space limitation which makes it difficult to meet demands for a wider foot space in front of the driver's seat for better driving maneuverability.
Recent motor vehicles incorporate various other mechanisms in combination with the steering shaft for better steering operation. Such mechanisms include a tilt mechanism for allowing the steering wheel to be tilted to a desired angle and a telescopic mechanism for adjusting the distance between the driver and the steering wheel. Since these mechanisms are mounted on the steering shaft, considerable efforts are required to make a layout in which to assemble the steering rotational speed sensor and these mechanisms neatly in a limited space on and around the steering shaft.